The present invention relates to an image reading apparatus, and to an image forming apparatus utilizing the image reading apparatus.
A document reading section (or an image reading apparatus), such as a digital copying machine, a facsimile, an image scanner and so on for example, utilizes an optical system to image a document on a light-receiving element (or a photoelectric conversion element), so as to convert an image of the document into an electrical signal. As a method of reading the document image in color, there has been proposed a method which converts information on colors of the document image into a signal, by subsequently lighting plural light sources provided in an illumination system and having different wavelength ranges, respectively, to read out the document image. There has been also a method which converts the color information of the document image, by utilizing a so-called three-line CCD (Charge-Coupled Device) as a light-receiving element, in which, for example, three lines of line sensors having red, green and blue filters, respectively, are aligned on a single chip, to perform color separation of the document image into three primary colors so as to read out the document image.
It has been known that an image reading apparatus of a digital copying machine applies an image process of pseudo-halftone processing to a photograph region, applies an image process of moire removing processing to a dot region, and applies an image process of sharpening processing, in which resolution is emphasized, to a line drawing region such as characters, in a case when a document image is mixed with a photograph, a dot image and a line drawing such as characters, in order to achieve high image quality in copying. Additionally, the most appropriate encoding type is selected for each of the regions to improve a compression rate of an image when the image is to be transmitted. In order to realize such image processes, the image reading apparatus of the digital copying machine performs an image process referred to as a so-called image region separation process, in which a character region and a design region (such as the photograph region and the dot region) in the image are separated at high precision, as preprocessing of such image processes.
In addition, as the image processes, a filter process or a spatial filter process has been generally performed for a digital image signal, for the purpose of improving sharpness, smoothness and so on of the image. In particular, a MTF (Modulation Transfer Function) compensation for improving the sharpness of characters and a smoothing process for suppressing the moire of a dot are essential processes in the image processor such as the digital copying machine which handle the image signal read by a scanning device.
Formerly, in the filter process performed in a color copying machine, a method of implementing the filter process to signals of three primary colors or the three primary colors plus a color of black (K=Keytone), for example RGB (Red, Green and Blue) or CMYK (Cyan, Magenta, Yellow, and Keytone), has been general. However, in recent years, a method in which at least one of luminance/color difference signals, value/chromaticity signals, and value/saturation/hue signals are used, and which implements the filter process to the signals in which characteristic of colors are considered, is becoming mainstream.
An advantage of the filter process to the luminance/color difference signals and so on is that filter processing, in which visual characteristics of humans such as high sensitivity for the luminance and low sensitivity for the color difference are considered, is possible. Another advantage is that adjustment of intensity of edge reinforcement, adjustment of the smoothing, and adjustment of degree of compensation of signals and so forth for the luminance signal and the color difference signal for example are easily realized according to a characteristic of an input image such as saturation.
Generally, it is required for a reading lens used in the image reading apparatus to have a high contrast in a high spatial frequency domain in an image plane, and at the same time, to have an aperture efficiency of substantially 100% up to a peripheral part of a field angle. Furthermore, in order to achieve decent reading of a colored document, it is necessary to match imaging positions of respective colors of red, green and blue in a direction of an optical axis on a light-receiving surface, and to perform a significantly fine compensation for chromatic aberration of the respective colors. Accordingly, it is necessary to design the reading lens such that a field curvature is suppressed to be extremely small and that an imaging performance in each image height from a part near the optical axis to a peripheral part becomes even.
Furthermore, in recent years, a bright lens having an F-number of about F4.2 has been demanded for the reading lens, in order to cope with a demand for improvement in productivity or speed-up in copying. In order to cope with the above demands, a Gauss type imaging lens having a four-group and six-piece structure, which is capable of suppressing generation of coma flare even when a relatively large diameter is employed and in which performance of the chromatic aberration compensation is also high, has been used as the imaging lens which satisfies performance and specification for coping with those demands.
For example, Japanese Patent Registration No. 2729039, No. 2790919, No. 3934197, and No. 3821929, as well as Japanese Patent Publication No. H09-304696 and No. 2001-166359 each discloses the invention in which a so-called anomalous dispersion glass, wherein partial dispersion deviation has a plus property, is used for at least one of a third lens and a fourth lens which are negative lenses, so as to better compensate chromatic aberration on an optical axis, in a case in which the Gauss type is used.
However, since the number of pieces of lens structuring the lens is six, which is large, the Gauss type has a drawback in that an external diameter of the lens becomes large, and has limitation or earlier restriction in miniaturization as well as cost reduction of the lens and a device utilizing such lens. Furthermore, the anomalous dispersion glass having relatively high dispersion (for example, Abbe number is in the vicinity of 40 to 45), which is generally used for the negative lens, has a problem in terms of fabrication such as easier generation of lens burn, and thus has a disadvantage in that fabrication costs are increased.
On the other hand, Japanese Patent Publication No. 2002-31753 and No. 2001-166359 each discloses the invention of a telephoto type having a reduced number of pieces of lens structuring a lens, i.e., having a four-group and four-piece structure. However, the telephoto type disclosed in Japanese Patent Publication No. 2002-31753 and No. 2001-166359 has an F-number of 6 or more, which is dark, and thus cannot cope with speed-up of copying.
In addition, Japanese Patent Registration No. 3856258 and Japanese Patent Publication No. H09-101452 each discloses the invention which achieves diameter increase in the telephoto type. However, although the invention disclosed in Japanese Patent Registration No. 3856258 has the F-number of 4.5 which is relatively large in a diameter, an amount of aberration is extremely large. Thus, the invention disclosed in Japanese Patent Registration No. 3856258 has a disadvantage in that it does not have imaging performance useable for high-density reading such as 600 dpi for example.
Also, while the invention disclosed in Japanese Patent Publication No. H09-101452 has the F-number of four which is large in the diameter, axial chromatic aberration of both a blue region and a red region is extremely large. Thus, the invention disclosed in Japanese Patent Registration No. H09-101452 has a disadvantage in that it does not have performance usable for a full color copying machine.